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Troilite spiders |
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| Meteorite, Willamette, IIIAB iron |
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| Figure 1. Scale bar 80 µm. |
| Meteorite, Willamette, IIIAB iron |
| Describing "spiders" with
meteorites would seem more appropriate for a SciFi novel or the opening of a
"B" movie. But no, "Troilite spiders" is a published term for the spidery shapes formed by the mineral troilite within some interior sections of the Willamette meteorite. They are also likely to be the key to understanding the fascinating sculptured appearance of this 14.5-ton iron meteorite. Troilite is an iron sulfide mineral (FeS) found in many iron meteorites. It is present as large nodules 1-3 cm across and the more minor lenticular/lamellar inclusions in Willamette. The spidery shape of these troilite bodies is a distinctive mark of Willamette. They are rare in occurrence but not unique to Willamette, having also been observed in the Willow Creek, IAB iron. Twice in its history, all of the troilite in Willamette was shock-melted. The spidery troilite appears to be the end product of a series of shock-altered transformational stages that began with the original lenticular and lamellae shapes. Numerous filaments radiate outward from the center of many troilite inclusions. In some, the filaments radiate through the kamacite, cutting into preexisting plessite fields. This physical description is clear and observable with microscopy. But how did these spiders occur? Willamette is the 10th largest known meteorite, weighing 31,107 lbs on a railroad scale in 1906 (Hovey 1906). Buchwald (1975) writes in his study of Willamette, "...the structure is complicated and seems to require a series of events for its formation." He proposes a 5-stage formative model: Stage 1. Ending the primary slow cooling period, Willamette would exhibit a medium Widmastatten structure with nodules and lenticular lenses of troilite. Stage 2. At this stage, a lot has to happen. It would require significant shock and reheating, shock melting the troilite, and recrystallizing the kamacite. In this stage, the troilite penetrated the kamacite and plessite fields from multiple directions. The kamacite recrystallized starting grain growth, with individual grains meeting along plessite fields and troilite filaments. The troilite is also recrystallized, and small particles of phosphides and taenite were absorbed into the recrystallization. Stage 3. This stage requires a lesser impact shock than stage 2, only sufficient enough for Neumann lines to form in the now recrystallized kamacite. Any reheating would have annealed and polygonized the kamacite, but this was below the temperature needed to recrystallize it a third time. Stage 4. Atmospheric passage. Willamette's terrestrial age is unknown but is believed to be tens of thousands of years. Weathering has removed all remnants of fusion crust and any heat-affected zones.The next part, Stage 5, pertains to the troilite spiders and may answer Willamette's most unusual appearance. Stage 5. The exterior of the 3m x 2m meteorite is covered with a labyrinth of holes and cavities, some 65cm in length and 45cm in depth. A photograph published by Ward (1904) shows these cavities sufficiently large and deep to enclose two children. See image. These cavities are not regmaglypts occurring during atmospheric passage. They are likely corrosion byproducts from tens of thousands of years of exposure in a humid Oregon Valley forest. The distinct spidery troilite filaments probably played a critical role in their development by creating lanes and diffusion paths through the kamacite, plessite, and taenite. These paths were likely studded with electrochemical cells fueled by dilute sulfuric acid from decomposing troilite (FeS). The troilite is likely secondary, forming through shock metamorphismm. The "spiders" are considered secondary. |
| Figure 2. Scale bar 120 µm. |
| Meteorite, Willamette, IIIAB |
| Figure 3. Scale bar 50 µm. |
| Meteorite, Willamette, IIIAB |
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| The main mass of Willamette is at the American Museum of Natural History, NY. |
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